At its most basic, a clause is a grammatical unit that contains a subject and a verb. Clauses differ from sentences (which always contain at least one clause but usually contain more) and phrases (which do not have both a subject and a verb).
What Are Defining Clauses?
A defining clause, sometimes called a restrictive clause, gives essential information that defines or identifies the person, place, or thing being referred to. Without this information, we’d struggle to understand the sentence’s meaning.
What Are Non-defining Clauses?
A non-defining clause (non-restrictive clause) provides additional information about someone, somewhere, or something, but this information is unnecessary for the sentence to make sense. If the clause is removed, the sentence would still be grammatically correct, and the meaning wouldn’t change much, although we’d have less detail.
Defining vs. Non-defining Clauses
So, defining clauses contain vital information, and non-defining clauses add non-essential information. Let’s compare the two.
The man who was wearing a blue suit sat down in front of me. Here, the phrase “who was wearing a blue suit” is essential to differentiate this man from the other men that this sentence implies are present.
The man, who was wearing a blue suit, sat down in front of me. Here, there is only one man, and he is the only one wearing a blue suit, but what he is wearing is not necessary to identify him.
A relative pronoun is a type of pronoun that marks a relative clause. The most common relative pronouns are that, which, when, who, whose, and where. These pronouns change depending on what you’re talking about and whether you are referring to the subject, the object, or the possessive.
Non-defining clauses never use that.
Defining: The hotel where I stayed overnight had a sauna.
Non-defining: Louisa, whose family is from Italy, is traveling home for the holidays.
We can sometimes omit the relative pronoun (in the cases of who, that, and which) if it is followed by a subject, but we usually can’t after a verb.
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This is the woman [that] I met while on holiday.
I took a pill, which helped with the pain, and went to bed.
We can’t omit the pronoun “which” because the sentence would sound disjointed.
You will probably have already noticed that non-defining clauses are separated from the rest of the sentence by commas because they don’t contain essential information.
Stratford-upon-Avon, which is the birthplace of William Shakespeare, is a village in the West Midlands.
Defining clauses don’t need commas.
Defining clauses in speech often use that instead of who, so “the people who spoke no English got lost in London” would become “the people that spoke no English got lost in London” in speech. This doesn’t happen for non-defining clauses because they don’t use “that.”
In non-defining clauses, we usually pause before and after the non-defining clause (usually where the commas are). We don’t do this for defining clauses.
So, the key points are:
Provides vital information necessary to the sentence
Adds information not necessary to the sentence
Who, whom, whose, where, which, that
Who, whom, whose, where, which
Can omit the pronoun if it is followed by a subject